ABSTRACT

One of several generalizations regarding the longevity of a typical estuary is that it will, ultimately, suffer destruction by infilling. While it is true that many estuaries have suffered this fate, natural events often intervene to prolong the existence of the site. For example, any rise of sea level will extend the estuary's existence, as will a decrease in the quantity of sediment being supplied to the estuary by rivers emptying into the basin. These phenomena must necessarily act over long time spans that require, at the minimum, centuries. One natural event that can duplicate the action of these long-term phenomena in a matter of hours, however, is the tropical cyclone. While it has long been known that hurricanes have the ability to markedly alter areas exposed above sea level, additional information is now available to demonstrate that they are similarly effective in causing submarine erosion and can scour and remove enormous quantities of sediment, even from well protected estuaries.

Apalachicola Bay, Florida, is one of the larger estuaries in the northern Gulf of Mexico and has one of the highest sedimentation rates of any estuary in the eastern United States. The massive influx of sediment from the Apalachicola River system has built an extensive delta system at the river's mouth. In the vicinity of the delta, sedimentation rates up to 10 mm/year have been measured. Analysis of bathymetric data dating back to the mid-1800's disclosed that the bay has been undergoing exceptionally rapid infilling which persists.

The passage of two hurricanes near the bay in 1985, however, generated high-velocity currents that scoured some 3.6 billion cubic feet of sediment from the bottom of the bay, completely changing the character of the texture, mineralogy, and heavy metal chemistry of the bottom sediments. Even more remarkable, perhaps, is the fact that over 83 million tons of sediment was carried by the currents out through one of the passes into the Gulf of Mexico. The removal of this quantity of sediment has thus returned the bay to bathymetric conditions existing in the early 1800's and significantly extended the life of the bay.